Abstract
The thermal decomposition of ulexite, NaCa[B5O6(OH)6] · 5H2O, monocrystals was investigated by thermal, X-ray, IR and optical microscopy methods at normal and elevated temperatures. It was found that the thermal decomposition has an intraframework character and proceeds in a few separate stages: 1) release of part of the molecular water coordinating the Ca and Na cations; this leads to a rearrangement of the ulexite structure; 2) release of the remaining molecular water and some part the OH groups, which causes breakdown of the ulexite structure, with the simultaneous crystallization of CaO · B2O3 and 2CaO·B2O3; 3) the slow release of the remaining OH groups up to 600°C, which causes decomposition of the borate rings and the crystallization of NaB3O5 and NaCaBO3.
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